1. Field of the Invention
The present invention relates to a process for preparing molded thermochromic polyvinyl chloride products which are highly sensitive to a change of temperature to undergo a reversible color change.
2. Description of the Prior Art
Polyvinyl chloride paste can be processed into a wide variety of products inexpensively by various methods, such as coating, casting, dipping and molding, with use of relatively simple equipment.
However, these products are simply colored with usual dyes or pigments and therefore still remain to be given a higher commercial value.
In recent years, it has become well known to form desired designs, markings or patterns on paper, cloth, leather or the like with various thermochromic materials, especially with an ink having high coloring ability and incorporating the three components of electron-donating chromogenic substance, electron-accepting substance and solvent in mixture so as to give color which reversibly changes with a change of ambient temperature. Attention has been directed to the use of such thermochromic materials for forming thermochromic patterns on articles prepared from polyvinyl chloride paste since the product is expected to have great industrial usefulness.
While the thermochromic material which is a mixture of the foregoing three components exhibits thermochromic properties only when the three components are strictly in a specified ratio, the three-component mixture has drawbacks when admixed with vinyl chloride plastisol. The three components are liable to physically flow out of the system owing to dissolving out or evaporation. The plasticizer, stabilizer, lubricant or the like contained in the paste will penetrate into the system to produce an adverse effect. Such phenomena become more pronounced at elevated temperatures, with the result that the product often fails to exhibit any color change in response to a change of temperature. Accordingly, it has been practically impossible to use thermochromic materials for molded vinyl chloride products.